Apparent resistivity is computed from the MT response tensor (colloquially referred to as the MT impedance) using
where $\mu_0$ is the permeability of free space, $\omega$ is the angular frequency in radians, and $T$ is period is seconds.
$Z_{ij}$ are the components of the MT response tensor.
Note that written in terms of the MT impedance $\mu_0 Z$, the formula for apparent resistivity is $\rho_i_j = |Z_i_j|^2 / (\omega\mu_0)$.
The units for apparent resistivity are [Ohm m].
See also MT impedance phase.
Chave, A.D. and Jones, A.G. eds., 2012. The magnetotelluric method: Theory and practice. Cambridge University Press.
Wight, D.E., 1987. Society of Exploration Geophysicists MT/EMAP Data Interchange Standard.